Simulating Tropical Clouds and Precipitation in Climate Models

Cloud-resolving models have difficulty representing the array of processes and scales in tropical storms.

The CSIRO (Commonwealth Scientific and Industrial Research Organization) Southern Surveyor docked prior to participating in the Tropical Warm Pool International Cloud Experiment in Darwin, Australia.

The Science

Eight Cloud Resolving Models from institutions in the USA, UK, and France simulated two weeks of active and suppressed monsoon conditions observed around the Department of Energy’s Atmospheric Radiation Measurement (ARM) Climate Research Facility site at Darwin, Australia.

The Impact

Simulations and observations often do not agree within experimental uncertainties indicating a need for more rigorous observationally based evaluation of the underlying properties of stratiform rain structures in simulations.

Summary

Tropical clouds and precipitation are components of the Earth's climate system and therefore critical to simulate faithfully in climate models. However, even the most detailed cloud-resolving models (CRMs) have difficulty representing the complex array of processes and scales that occur in tropical storms, including wind patterns, cloud particles, rain, and ice, in part because many key processes are not well understood and in part because the complexity is difficult to represent (such as the variety of ice particle properties). A unique aspect of this study focused on the evaluation of the overall structure of CRM-simulated cloud systems. The analysis was applied in an identical fashion to both observations and simulations to identify the heavily raining updraft regions (so-called convective areas) and less violent and lighter-raining broad regions (so-called stratiform areas). This allowed direct comparison of convective and stratiform cloud features in observations and simulations. It was found that convective areas were quite similar to the observations in terms of timing, but the simulated regions were always larger than observed in every simulation, by more than observational uncertainty could explain. Stratiform areas were far more variable in timing and extent. The wide spreads of predicted stratiform cloud properties and their close association with radiative fluxes, which are important to climate in the tropics, indicate that there is a need for more rigorous observationally based evaluation of the underlying properties of stratiform rain structures in simulations.

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